1 // SPDX-License-Identifier: GPL-2.0
2
3 //! A kernel mutex.
4 //!
5 //! This module allows Rust code to use the kernel's `struct mutex`.
6
7 /// Creates a [`Mutex`] initialiser with the given name and a newly-created lock class.
8 ///
9 /// It uses the name if one is given, otherwise it generates one based on the file name and line
10 /// number.
11 #[macro_export]
12 macro_rules! new_mutex {
13 ($inner:expr $(, $name:literal)? $(,)?) => {
14 $crate::sync::Mutex::new(
15 $inner, $crate::optional_name!($($name)?), $crate::static_lock_class!())
16 };
17 }
18 pub use new_mutex;
19
20 /// A mutual exclusion primitive.
21 ///
22 /// Exposes the kernel's [`struct mutex`]. When multiple threads attempt to lock the same mutex,
23 /// only one at a time is allowed to progress, the others will block (sleep) until the mutex is
24 /// unlocked, at which point another thread will be allowed to wake up and make progress.
25 ///
26 /// Since it may block, [`Mutex`] needs to be used with care in atomic contexts.
27 ///
28 /// Instances of [`Mutex`] need a lock class and to be pinned. The recommended way to create such
29 /// instances is with the [`pin_init`](crate::pin_init) and [`new_mutex`] macros.
30 ///
31 /// # Examples
32 ///
33 /// The following example shows how to declare, allocate and initialise a struct (`Example`) that
34 /// contains an inner struct (`Inner`) that is protected by a mutex.
35 ///
36 /// ```
37 /// use kernel::sync::{new_mutex, Mutex};
38 ///
39 /// struct Inner {
40 /// a: u32,
41 /// b: u32,
42 /// }
43 ///
44 /// #[pin_data]
45 /// struct Example {
46 /// c: u32,
47 /// #[pin]
48 /// d: Mutex<Inner>,
49 /// }
50 ///
51 /// impl Example {
52 /// fn new() -> impl PinInit<Self> {
53 /// pin_init!(Self {
54 /// c: 10,
55 /// d <- new_mutex!(Inner { a: 20, b: 30 }),
56 /// })
57 /// }
58 /// }
59 ///
60 /// // Allocate a boxed `Example`.
61 /// let e = Box::pin_init(Example::new(), GFP_KERNEL)?;
62 /// assert_eq!(e.c, 10);
63 /// assert_eq!(e.d.lock().a, 20);
64 /// assert_eq!(e.d.lock().b, 30);
65 /// # Ok::<(), Error>(())
66 /// ```
67 ///
68 /// The following example shows how to use interior mutability to modify the contents of a struct
69 /// protected by a mutex despite only having a shared reference:
70 ///
71 /// ```
72 /// use kernel::sync::Mutex;
73 ///
74 /// struct Example {
75 /// a: u32,
76 /// b: u32,
77 /// }
78 ///
79 /// fn example(m: &Mutex<Example>) {
80 /// let mut guard = m.lock();
81 /// guard.a += 10;
82 /// guard.b += 20;
83 /// }
84 /// ```
85 ///
86 /// [`struct mutex`]: srctree/include/linux/mutex.h
87 pub type Mutex<T> = super::Lock<T, MutexBackend>;
88
89 /// A kernel `struct mutex` lock backend.
90 pub struct MutexBackend;
91
92 // SAFETY: The underlying kernel `struct mutex` object ensures mutual exclusion.
93 unsafe impl super::Backend for MutexBackend {
94 type State = bindings::mutex;
95 type GuardState = ();
96
97 unsafe fn init(
98 ptr: *mut Self::State,
99 name: *const core::ffi::c_char,
100 key: *mut bindings::lock_class_key,
101 ) {
102 // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
103 // `key` are valid for read indefinitely.
104 unsafe { bindings::__mutex_init(ptr, name, key) }
105 }
106
107 unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
108 // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
109 // memory, and that it has been initialised before.
110 unsafe { bindings::mutex_lock(ptr) };
111 }
112
113 unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
114 // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
115 // caller is the owner of the mutex.
116 unsafe { bindings::mutex_unlock(ptr) };
117 }
118 }
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